1. Field of the Invention
The present invention relates to a lens, and in particular to a wide-angle lens, of which the overall length is shortened for use in an imaging device.
2. Description of Prior Art
Lenses for imaging devices, such as video cameras and digital cameras, are classified by the focal length as standard lenses, wide-angle lenses and telephoto lenses. Generally, the wide-angle lens is a short zoom lens, both an angle of view and a shoot scope of which become larger with decreasing of the lens focal length. The wide-angle lens can be used to photograph a larger scene or a whole scene in a limited distance. For example, a video camera adopting the wide-angle lens is adapted for use in network video and photographs all the scenes located at a user's end, thereby realizing the real-time network communication.
As an image pick-up device, conventional optical spherical lenses have been widely adopted in optical industry field because of easy machining. However, the spherical lenses have the disadvantages of uneasy correction of spherical aberration and astigmatic aberration. To improve the above-mentioned drawbacks existing in the conventional spherical lenses, aspheric lenses have been used to obtain better image quality.
U.S. Pat. No. 6,982,834 B2 discloses a wide-angle zoom lens including at least one aspheric lens surface. This conventional wide-angle zoom lens includes a first lens group having a negative refractive power, a second lens group having a positive refractive power and a third lens group having a positive refractive power. The conventional wide-angle zoom lens includes at least one aspheric lens surface. When a difference value between the Abbe numbers of the first and second lens groups is larger than 20.5 and a half angle of view of the zoom lens is located between 36.0 and 41.0, the zoom lens can obtain better image quality.
U.S. Pat. No. 6,825,993 B2 discloses a wide-angle lens having aspheric surface. This conventional wide-angle lens includes, in order from the object side, a negative first plastic lens element, a positive second plastic lens element, a positive cemented doublet element and a positive lens element. All lens elements of the wide-angle lens except the cemented doublet element have an aspheric surface. When a ratio of the distance between the first plastic lens and the second plastic lens to the focal length of the wide-angle lens is 0.5 to 1.0, the wide-angle lens can obtain better image quality.
It is known that any wide-angle lens can improve the optical aberration not only by adopting the aspheric surface and the cemented doublet element, but also by satisfying specified conditions.
With developing of image devices and network video, there is a great demand for wide-angle lenses. Conventional wide-angle lenses as mentioned above can not satisfy the new requirement of image devices having new performances. Accordingly, how to reduce the size and weight of the whole lens system while maintaining high optical performance is a problem encountered by a lens designer.